docs(claude-code): document pre_install_script for module dependency ordering (#613)

## Summary

Clarifies that the existing `pre_install_script` variable can be used to
handle dependencies between modules during workspace startup.

## Problem

When using multiple startup modules (e.g., git-clone and claude-code),
there's a race condition where scripts execute in parallel. Module
dependencies need to be managed, such as ensuring git-clone completes
before Claude Code tries to access a workdir.

## Solution

The existing `pre_install_script` variable already provides this
capability. Updated documentation to clarify this use case.

## Example

```hcl
module "claude-code" {
  source = "registry.coder.com/coder/claude-code/coder"
  
  workdir = "/path/to/repo"
  
  # Wait for git-clone to complete before starting
  pre_install_script = <<-EOT
    #!/bin/bash
    set -e
    while [ ! -f /tmp/.git-clone-complete ]; do
      sleep 1
    done
  EOT
}
```

Resolves issue #609.

Co-authored-by: Jason Barnett <Jason.Barnett@altana.ai>
Co-authored-by: DevCats <christofer@coder.com>

registry/coder/modules/claude-code/main.tf

@@ -67,7 +67,7 @@ variable "cli_app_display_name" {
 
 variable "pre_install_script" {
   type        = string
-  description = "Custom script to run before installing Claude Code."
+  description = "Custom script to run before installing Claude Code. Can be used for dependency ordering between modules (e.g., waiting for git-clone to complete before Claude Code initialization)."
   default     = null
 }